Method of manufacturing a flexible circuit electrode array

ABSTRACT

Polymer materials form electrode array bodies for neural stimulation, especially for retinal stimulation to create vision. The method lays down a polymer layer. Apply a metal layer to the polymer and pattern to create electrodes and leads. Apply a second polymer layer over the metal layer and pattern to leave openings for electrodes. The array and its supply cable are a single body. A method for manufacturing a flexible circuit electrode array, is: deposit a metal trace layer on an insulator polymer base layer; apply a layer of photoresist on the metal trace layer and pattern the metal trace layer and form metal traces on the insulator polymer base layer; activate the insulator polymer base layer and deposit a top insulator polymer layer and form a single insulating polymer layer with the base insulator polymer layer; wherein the insulator polymer layers are heated at 80-150° C. and then at 230-350° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Application No. 60/772,099,“Flexible Circuit Electrode Array and Method of Manufacturing the Same,”filed Feb. 10, 2006, the disclosure of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant No.R24EY12893-01, which has been awarded by the National Institutes ofHealth. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally directed to a flexible circuitelectrode array especially for biomedical implants, especiallyimplantable medical devices, such as retinal prosthesis and a method ofmanufacturing the flexible circuit electrode array.

2. Description of the Related Art

In U.S. Pat. No. 3,699,970 “Striate Cortex Stimulator” to Giles SkeyBrindley et al. an implantable device is disclosed comprising aplurality of electrodes for stimulating the striate cortex.

In U.S. Pat. No. 4,487,652 “Slope Etch of Polyimide” to Carl W. Amgren asemiconductor having an insulating layer overlying a metal layer isdisclosed, wherein the insulator comprises an upper oxide layer, anintermediate polyimide layer, and a lower oxide layer in contact withthe metal layer, a method for etching a via from an upper surface of thepolyimide layer to the metal layer comprising the steps of applyingphotoresist; etching an opening from an upper surface of the photoresistlayer to the upper oxide layer at a location for forming the via so thatan upper surface of the upper oxide layer is exposed at the vialocation; heating the photoresist to cause a more gradual slope of thephotoresist layer from the upper surface of the upper oxide layer at thevia location to the upper surface of the photoresist layer; applyingreactive ion etchant with a predetermined selectivity betweenphotoresist and oxide to transfer the slope of the photoresist layer tothe upper oxide layer at a predetermined ratio; and applying a reactiveion etchant with a predetermined selectivity between oxide and polyimideto transfer the slope of the upper oxide layer to the polyimide layer ata predetermined ratio, whereby the lower oxide layer is simultaneouslyetched to expose the metal layer at the via location.

In U.S. Pat. No. 4,573,481 “Implantable Electrode Array” to Leo A.Bullara an electrode assembly for surgical implantation on a nerve ofthe peripheral nerve system is disclosed.

In U.S. Pat. No. 4,628,933 “Method and Apparatus for Visual Prosthesis”to Robin P. Michelson a visual prosthesis for implantation in the eye inthe optical pathway thereof is disclosed.

In U.S. Pat. No. 4,837,049 “Method of Making an Electrode Array” toCharles L. Byers et al. a very small electrode array which penetratesnerves for sensing electrical activity therein or to provide electricalstimulation is disclosed.

In U.S. Pat. No. 4,996,629 “Circuit Board with Self-SupportingConnection Between Sides” to Robert A. Christiansen et al. a coppersupporting sheet is disclosed having vias for connecting semiconductorchips to surface mount components. A laminate of polyimide has viascorresponding to the supporting layer vias with copper covering thosevias.

In U.S. Pat. No. 5,108,819 “Thin Film Electrical Component” to James W.Heller a thin film electrical component is disclosed comprising a rigidglass carrier plate, a substrate bonded to the rigid glass carrierplate, the substrate comprising a polyimide establishing a bond with therigid glass carrier plate that is broken upon immersion of the substrateand the rigid glass carrier plate in one of a hot water bath and a warmtemperature physiologic saline bath to release the polymer fromattachment to the rigid glass carrier plate, and means for providing anelectrical circuit, the providing means being bonded to the substrateand undisrupted during release of the substrate from attachment to therigid glass carrier plate.

In U.S. Pat. No. 5,109,844 “Retinal Microstimulation” to Eugene de JuanJr. et al. a method for stimulating a retinal ganglion cell in a retinawithout penetrating the retinal basement membrane at the surface of theretina is disclosed.

In U.S. Pat. No. 5,178,957 “Noble Metal-Polymer Composites and FlexibleThin-Film Conductors Prepared Therefrom” to Vasant V. Kolpe a compositearticle is disclosed comprising a polymeric support selected from thegroup consisting of a polyimide, polyethylene terephthalate, andpolyester-ether block copolymer having a noble metal deposited directlyonto at least one surface, wherein said deposited metal exhibits a peelforce of at least about 0.05 kg per millimeter width after 24 hourboiling saline treatment.

In U.S. Pat. No. 5,215,088 “Three-Dimensional Electrode Device” toRichard A. Norman et al. a three-dimensional electrode device forplacing electrodes in close proximity to cell lying at least about 1000microns below a tissue surface is disclosed.

In U.S. Pat. No. 5,935,155 “Visual Prosthesis and Method of Using Same”to Mark S. Humayun et al. a visual prosthesis is disclosed comprising acamera for receiving a visual image and generating a visual signaloutput, retinal tissue stimulation circuitry adapted to be operativelyattached to the user's retina, and wireless communication circuitry fortransmitting the visual signal output to the retinal tissue stimulationcircuitry within the eye.

In U.S. Pat. No. 6,071,819 “Flexible Skin Incorporating MEMS Technology”to Yu-Chong Tai a method of manufacturing a flexible microelectronicdevice is disclosed comprising first etching a lower side of a waferusing a first caustic agent; depositing a first layer of aluminum on anupper side of the wafer; patterning the first layer of aluminum;depositing a first layer of polyimide on the upper side of the wafer,covering the first layer of aluminum; depositing a second layer ofaluminum on the upper side of the wafer, covering the first layer ofpolyimide; depositing a second layer of polyimide on the upper side ofthe wafer, covering the second layer of aluminum; depositing a thirdlayer of aluminum on the lower side of the wafer; patterning the thirdlayer of aluminum; second etching the lower side of the wafer using thethird layer of aluminum as a mask and the first layer of aluminum as anetch stop and using a less caustic agent than said first caustic agent,such that the wafer is divided into islands with gaps surrounding eachisland; and depositing a third layer of polyimide on the lower side ofthe wafer, such that the gaps are at least partially filled.

In U.S. Pat. No. 6,324,429 “Chronically Implantable Retinal Prosthesis”to Doug Shire et al. an apparatus is disclosed which is in contact withthe inner surface of the retina and electrically stimulates at least aportion of the surface of the retina.

In U.S. Pat. No. 6,374,143 “Modiolar Hugging Electrode Array” to PeterG. Berrang et al. a cochlear electrode array for stimulating auditoryprocesses is disclosed.

In U.S. Pat. No. 6,847,847 “Retina Implant Assembly and Methods forManufacturing the Same” to Wilfried Nisch et al. a retina implant isdisclosed comprising a chip in subretinal contact with the retina and areceiver coil for inductively coupling there into electromagneticenergy.

In U.S. Pat. No. 6,970,746 A1, “Microcontact structure forneuroprostheses for implantation on nerve tissue and method therefore”to Rolf Eckmiller et al. a four layer microcontact structure isdisclosed in which the active connection between the microcontactstructure and the nerve tissue is brought about by electricalstimulation. The layer adjacent to the nerve tissue to be stimulated iscomposed of the polymer polyimide and contains penetrating electrodesmade of platinum which forms the adjoining layer. There follows afurther layer of the polyimide and a layer of the polymer polyurethane.Polyurethane has the property of thermal expansion relative topolyimide.

In U.S. Pat. No. 6,976,998 A1 “Minimal Invasive Retinal Prosthesis” toJohn F. Rizzo et al. a retinal prosthesis is disclosed comprising an RFcoil attached to the outside of and moving with an eye to receive powerfrom an external power source; electronic circuitry attached to andmoving with the eye and electrically connected to the RF coil; a lightsensitive array electrically connected to the electronic circuitry andlocated within the eye for receiving incident light and for generatingan electrical signal in response to the incident light; and astimulating array abutting a retina of the eye and electricallyconnected to the electronic circuitry to stimulate retinal tissue inresponse to the electrical signal from the light sensitive array. Asupporting silicone substrate has a polyimide layer spun onto itssurface and cured. The copper or chrome/gold conducting layer is thenadded and patterned using wet chemical etching or a photoresist lift-offprocess. Next, a second polyimide layer is spun on, and the regionswhere circuit components are to be added are exposed by selective dryetching or laser ablation of the upper polyimide layer in the desiredareas. Finally, the completed components are removed from theirsupporting substrate.

Polyimide, also known as PI, has been mass-produced since 1955. It isused in bearing materials, thrust washers, and semiconductor waferclamps. It has high impact and dielectric strength, high heat resistanceand a low coefficient of thermal expansion and excellent mechanical,thermal, and electrical properties. Polyimide is typically applied inliquid form, and then thermally cured into a film or layer with thedesired properties. The film can be patterned using photographic orother processes. Microelectronic applications include stress buffer,passivation layer, chip bonding, and interlayer dielectric.

Eugene de Juan Jr. et al. at Duke University Eye Center inserted retinaltacks into retinas in an effort to reattach retinas that had detachedfrom the underlying choroid, which is the source of blood supply for theouter retina and thus the photoreceptors. See for example E. de JuanJr., et al., “Retinal tacks”, Am J. Ophthalmol. 1985 Mar. 15; 99(3):272-4.

Hansjoerg Beutel et al. at the Fraunhofer Institute for BiomedicalEngineering IBMT demonstrated the bonding of a gold ball by force,temperature, and ultrasound onto an aluminum metal layer. See forexample Hansjoerg Beutel, Thomas Stieglitz, Joerg-Uwe Meyer: “VersatileMicroflex-Based Interconnection Technique,” Proc. SPIE Conf. on SmartElectronics and MEMS, San Diego, Cal., March 1998, vol. 3328, pp174-182. A robust bond can be achieved in this way. However,encapsulation proves difficult to effectively implement with thismethod. Gold, while biocompatible, is not completely stable under theconditions present in an implant device since it dissolves byelectromigration when implanted in living tissue and subject to anelectric current. See for example Marcel Pourbaix: “Atlas ofElectrochemical Equilibria in Aqueous Solutions”, National Associationof Corrosion Engineers, Houston, 1974, pp 399-405.

A system for retinal stimulation comprising a polyimide-based electrodesbeing coated with platinum black are described by Andreas Schneider andThomas Stieglitz. See for example Andreas Schneider, Thomas Stieglitz:“Implantable Flexible Electrodes for Functional Electrical Stimulation”,Medical Device Technology, 2004.

A process for activating a base polyimide layer prior to applying a toppolyimide layer is described by Balasubrahmanyan Ganesh, who suggests toclean, roughen, and oxygenate the base polyimide by using reactive ionetching (RIE) in oxygen plasma for 10 s at 50 W, and 800 mTorr pressurein an Oxford Plasmalab-80 Plus system. The top polyimide layer is thenspun-on immediately after the plasma after the plasma roughening and thewafer is set aside for 45 min. See for example Balasubrahmanyan Ganesh:“A Polyimide Ribbon Cable for Neural Recording and Stimulation Systems”,a Thesis for the Degree of Master of Science, Department of MaterialsScience and Engineering, The University of Utah, March 1998.

A process for activating a base polyimide layer prior to applying a toppolyimide layer is described by Nancy Stoffel, Crystal Zhang and EdwardJ. Kramer. Stoffel et al. suggest using wet chemical treatments tohydrolyze the films according to a method reported by K. W. Lee et al.(1990). Stoffel et al. found solutions of both 1 M KOH and 1 Mtetramethyl ammonium hydroxide to be effective for polyimide films.Solutions of 0.2 M HCl and acetic acid were used and found to beeffective for converting polyamate salt into a polyamic acid. See forexample Nancy Stoffel, Crystal Zhang and Edward J. Kramer: “Adhesion ofPolyimide Laminates”, Application of Fracture Mechanics in ElectronicPackaging and Materials, ASME, EEP-Vol. 11/MD-Vol. 64, pp. 79-84, 1995.

BRIEF SUMMARY OF THE INVENTION

The flexible circuit electrode array is electrically coupled by aflexible circuit cable, which pierces the sclera and is electricallycoupled to an electronics package. The flexible circuit electrode arrayand the flexible circuit cable are formed of a single body.

The pressure applied against the retina, or other neural tissue, by anelectrode array is critical. Low pressure causes increased electricalresistance between the array and retina, along with electric fielddispersion. High pressure may block blood flow within the retina causinga condition similar to glaucoma. Common flexible circuit fabricationtechniques such as photolithography generally require that a flexiblecircuit electrode array be made flat. Since the retina is spherical, aflat array will necessarily apply more pressure near its edges, than atits center. The edges of a flexible circuit polymer array may be quitesharp and cut the delicate retinal tissue. Most polymers can be curvedwhen heated in a mold. By applying the right amount of heat to acompleted array, a curve can be induced matching the curve of theretina. With a thermoplastic polymer such as liquid crystal polymer, itmay be further advantageous to repeatedly heat the flexible circuit inmultiple molds, each with a decreasing radius.

The present invention provides a method for manufacturing a flexiblecircuit electrode array, comprising:

a) depositing a metal trace layer on an insulator polymer base layer;

b) applying a layer of photoresist on said metal trace layer andpatterning said metal trace layer and forming metal traces on saidinsulator polymer base layer; and

c) activating said insulator polymer base layer and depositing a topinsulator polymer layer and forming one single insulating polymer layerwith said base insulator polymer layer; wherein the insulator polymerlayers were treated at a temperature from 80-150° C. and then at atemperature from 250-350° C.

The present invention provides a flexible circuit electrode array withexcellent adhesion and insulating properties of a polymer insulatorreached by a new technique of activation of a base polymer layer priorto applying a top polymer layer wherein both polymer layers result inone polymer layer. The adhesion and insulating properties are furtherimproved by applying a top metal layer on the electrode layer as anadhesion aid to the polymer.

The method of the present invention solves the long term problem of weekadhesion between the insulator polymer base layer and the top insulatorpolymer layer. The method of the present invention provides an excellentadhesion between the polymer base layer and the polymer top layer and anexcellent insulation of the trace metal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts a perspective view of the implanted portion of thepreferred retinal prosthesis including a twist in the array to reducethe width of a scleratomy and a sleeve to promote sealing of thescleratomy.

FIG. 2 depicts a perspective view of the implanted portion of theretinal prosthesis showing the fan tail in more detail.

FIGS. 3 a-3 e depicts a perspective view of molds for forming theflexible circuit array in a curve.

FIG. 4 depicts an alternative view of the invention with ribs to helpmaintain curvature and prevent retinal damage.

FIG. 5 depicts a top view of a body comprising a flexible circuitelectrode array, a flexible circuit cable and a bond pad before it isfolded and attached to the implanted portion.

FIG. 6 depicts a top view of a body comprising a flexible circuitelectrode array, a flexible circuit cable and a bond pad after it isfolded.

FIG. 7 depicts a top view of a body comprising a flexible circuitelectrode array, a flexible circuit cable and a bond pad after it isfolded with a protective skirt.

FIG. 8 depicts a cross-sectional view of a flexible circuit array with aprotective skirt bonded to the back side of the flexible circuit array.

FIG. 9 depicts a cross-sectional view of a flexible circuit array with aprotective skirt bonded to the front side of the flexible circuit array.

FIG. 10 depicts a cross-sectional view of a flexible circuit array witha protective skirt bonded to the back side of the flexible circuit arrayand molded around the edges of the flexible circuit array.

FIG. 11 depicts a cross-sectional view of a flexible circuit array witha protective skirt bonded to the back side of the flexible circuit arrayand molded around the edges of the flexible circuit array and flush withthe front side of the array.

FIG. 12 depicts a top view on a supporting glass plate substrate whichis marked with a batch and plate identification code.

FIG. 13 depicts a cross-sectional view of a layer of polyimide which isapplied onto the front side of the glass plate and cured.

FIG. 14 depicts a cross-sectional view of layer deposition oftitanium/platinum/titanium thin film stack from which conductor tracesare patterned.

FIG. 15 depicts a cross-sectional view of photoresist layer depositionon the titanium/platinum/titanium thin film stack after the photoresistlayer is exposed to irradiation through a mask and the radiated areas ofthe photoresist layer are removed during photolithographic processing toselectively mask titanium/platinum/titanium thin film stack in areaswhere conductor traces are to remain.

FIG. 16 depicts a cross-sectional view of the layer structure after theremoval of the top layer from titanium/platinum/titanium trace metalstack in areas exposed during photolithography.

FIG. 17 depicts a cross-sectional view of the layer structure after theremoval of the middle layer from titanium/platinum/titanium trace metalstack in areas exposed during photolithography.

FIG. 18 depicts a cross-sectional view of the layer structure after theremoval of residual photoresist from trace metal surface.

FIG. 19 depicts a cross-sectional view of the layer structure after theremoval of bottom layer from titanium/platinum/titanium trace metalstack in areas exposed during photolithography and preceding etches.

FIG. 20 depicts a cross-sectional view of the layer structure afteractivating and roughening and partial removal of base polyimide surfacelayer in all areas not covered by trace metal conductors.

FIG. 21 depicts a cross-sectional view of the layer structure after alayer of polyimide is spun over the underlying structures and cured.

FIG. 22 depicts a cross-sectional view of the layer structure aftercuring the polyimide and after depositing an aluminum thin film layer ontop of the polyimide layer that will serve to define openings in the toppolyimide insulation.

FIG. 23 depicts a cross-sectional view of the layer structure after aphotoresist layer is deposited on the aluminum layer and exposed toirradiation through a mask and the radiated areas of the photoresistlayer are removed during photolithographic processing to selectivelymask aluminum thin film layer in areas that are to remain.

FIG. 24 depicts a cross-sectional view of the layer structure after theremoval of aluminum layer in areas exposed during photolithography andpreceding etches.

FIG. 25 depicts a cross-sectional view of the layer structure after theremoval of top polyimide to create vias to trace metal in areas definedby aluminum etch mask.

FIG. 26 depicts a cross-sectional view of the layer structure after theremoval of the remaining photoresist from surface.

FIG. 27 depicts a cross-sectional view of the layer structure after theremoval of remaining aluminum mask layer.

FIG. 28 depicts a cross-sectional view of the layer structure after theremoval of exposed top titanium layer in areas opened through the toppolyimide.

FIG. 29 depicts a cross-sectional view of the layer structure after thesingulation of arrays to create individual parts prior to removal fromthe supporting glass plate substrate.

FIG. 30 depicts a cross-sectional view of the layer structure after thedeposition of additional material on electrode sites which depicts apart of the flexible circuit electrode array.

FIG. 31 depicts a top view of the flexible circuit electrode array.

FIG. 32 depicts a perspective view of a part of the flexible circuitelectrode array.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a perspective view of the implanted portion of thepreferred retinal prosthesis. A flexible circuit electrode array 10 ismounted by a retinal tack or similar means to the epiretinal surface.The flexible circuit electrode array 10 is electrically coupled by aflexible circuit cable 12, which pierces the sclera and is electricallycoupled to an electronics package 14, external to the sclera.

The electronics package 14 is electrically coupled to a secondaryinductive coil 16. Preferably the secondary inductive coil 16 is madefrom wound wire. Alternatively, the secondary inductive coil 16 may bemade from a flexible circuit polymer sandwich with wire traces depositedbetween layers of flexible circuit polymer. The electronics package 14and secondary inductive coil 16 are held together by a molded body 18.The molded body 18 may also include suture tabs 20. The molded body 18narrows to form a strap 22 which surrounds the sclera and holds themolded body 18, secondary inductive coil 16, and electronics package 14in place. The molded body 18, suture tabs 20 and strap 22 are preferablyan integrated unit made of silicone elastomer. Silicone elastomer can beformed in a pre-curved shape to match the Curvature 40 of a typicalsclera. However, silicone remains flexible enough to accommodateimplantation and to adapt to variations in the Curvature 40 of anindividual sclera. The secondary inductive coil 16 and molded body 18are preferably oval shaped. A strap 22 can better support an oval shapedcoil 16.

The implanted portion of the retinal prosthesis may include theadditional feature of a gentle twist or fold 48 in the flexible circuitcable 12, where the flexible circuit cable 12 passes through the sclera(scleratomy). The twist 48 may be a simple sharp twist, or fold; or itmay be a longer twist, forming a tube. While the tube is rounder, itreduces the flexibility of the flexible circuit cable 12. A simple foldreduces the width of the flexible circuit cable 12 with only minimalimpact on flexibility.

Further, silicone or other pliable substance may be used to fill thecenter of the tube or fold 48 formed by the twisted flexible circuitcable 12. Further it is advantageous to provide a sleeve or coating 50that promotes healing of the scleratomy. Polymers such as polyimide,which may be used to form the flexible circuit cable 12 and flexiblecircuit electrode array 10, are generally very smooth and do not promotea good bond between the flexible circuit cable 12 and scleral tissue. Asleeve or coating 50 of polyester, collagen, silicone, Gore-Tex® orsimilar material would bond with scleral tissue and promote healing. Inparticular, a porous material will allow scleral tissue to grow into thepores promoting a good bond.

The entire implant is attached to and supported by the sclera. An eyemoves constantly. The eye moves to scan a scene and also has a jittermotion to improve acuity. Even though such motion is useless in theblind, it often continues long after a person has lost their sight. Byplacing the device under the rectus muscles with the electronics package14 in an area of fatty tissue between the rectus muscles, eye motiondoes not cause any flexing which might fatigue, and eventually damage,the device.

Human vision provides a field of view that is wider than it is high.This is partially due to fact that we have two eyes, but even a singleeye provides a field of view that is approximately 90° high and 140° to160° degrees wide. It is therefore, advantageous to provide a flexiblecircuit electrode array 10 that is wider than it is tall. This isequally applicable to a cortical visual array. In which case, the widerdimension is not horizontal on the visual cortex, but corresponds tohorizontal in the visual scene.

FIG. 2 shows a side view of the implanted portion of the retinalprosthesis, in particular, emphasizing the fan tail 24. When implantingthe retinal prosthesis, it is necessary to pass the strap 22 under theeye muscles to surround the sclera. The secondary inductive coil 16 andmolded body 18 must also follow the strap 22 under the lateral rectusmuscle on the side of the sclera. The implanted portion of the retinalprosthesis is very delicate. It is easy to tear the molded body 18 orbreak wires in the secondary inductive coil 16. In order to allow themolded body 18 to slide smoothly under the lateral rectus muscle, themolded body 18 is shaped in the form of a fan tail 24 on the endopposite the electronics package 14.

The flexible circuit electrode array 10 is a made by the followingprocess. First, a layer of polymer is applied to a supporting substrate(not part of the array) such as glass. The polymer layer or films of thepresent invention can be made, for example, any one of the variouspolyfluorocarbons, polyethylene, polypropylene, polyimide, polyamide,silicone or other biologically inert organic polymers. Layers may beapplied by spinning, meniscus coating, casting, sputtering or otherphysical or chemical vapor deposition, or similar process. Subsequently,a metal layer is applied to the polymer. The metal is patterned byphotolithographic process. Preferably, a photoresist is applied andpatterned by photolithography followed by a wet etch of the unprotectedmetal. Alternatively, the metal can be patterned by lift-off technique,laser ablation or direct write techniques.

It is advantageous to make the metal thicker at the electrode and bondpad to improve electrical continuity. This can be accomplished throughany of the above methods or electroplating. Then, the top layer ofpolymer is applied over the metal. Openings in the top layer forelectrical contact to the electronics package 14 and the flexiblecircuit electrode array 10 may be accomplished by laser ablation orreactive ion etching (RIE) or photolithograph and wet etch. Making theelectrode openings in the top layer smaller than the electrodes promotesadhesion by avoiding delaminating around the electrode edges.

The pressure applied against the retina by the flexible circuitelectrode array 10 is critical. Too little pressure causes increasedelectrical resistance between the array and retina. Common flexiblecircuit fabrication techniques such as photolithography generallyrequire that a flexible circuit electrode array 10 be made flat. Sincethe retina is spherical, a flat array will necessarily apply morepressure near its edges, than at its center. With most polymers, it ispossible to curve them when heated in a mold. By applying the rightamount of heat to a completed array, a curve can be induced that matchesthe curve of the retina. To minimize warping, it is often advantageousto repeatedly heat the flexible circuit in multiple molds, each with adecreasing radius. FIG. 3 illustrates a series of molds according to thepreferred embodiment. Since the flexible circuit will maintain aconstant length, the curvature 30 must be slowly increased along thatlength. As the curvature 30 decreases in successive molds (FIGS. 3 a-3e) the straight line length between ends 32 and 34, must decrease tokeep the length along the curvature 30 constant, where mold 3Eapproximates the curvature 30 of the retina or other desired neuraltissue. The molds provide a further opening 36 for the flexible circuitcable 12 of the array to exit the mold without excessive curvature.

It should be noted that suitable polymers include thermoplasticmaterials and thermoset materials. While a thermoplastic material willprovide some stretch when heated a thermoset material will not. Thesuccessive molds are, therefore, advantageous only with a thermoplasticmaterial. A thermoset material works as well in a single mold as it willwith successive smaller molds. It should be noted that, particularlywith a thermoset material, excessive curvature 30 in three dimensionswill cause the polymer material to wrinkle at the edges. This can causedamage to both the array and the retina. Hence, the amount of curvature30 is a compromise between the desired curvature, array surface area,and the properties of the material.

Referring to FIG. 4, the edges of the polymer layers are often sharp.There is a risk that the sharp edges of a flexible circuit will cut intodelicate retinal tissue. It is advantageous to add a soft material, suchas silicone, to the edges of a flexible circuit electrode array 10 toround the edges and protect the retina. Silicone around the entire edgemay make the flexible circuit less flexible. It is advantageous toprovide silicone bumpers or ribs to hold the edge of the flexiblecircuit electrode array 10 away from the retinal tissue. Curvature 40fits against the retina. The leading edge 44 is most likely to causedamage and is therefore fit with molded silicone bumper. Also, edge 46,where the array lifts off the retina can cause damage and should be fitwith a bumper. Any space along the side edges of curvature 40 may causedamage and may be fit with bumpers as well. It is also possible for theflexible circuit cable 12 of the electrode array to contact the retina.It is, therefore, advantageous to add periodic bumpers along the cable12.

It is also advantageous to create a reverse curve or service loop in theflexible circuit cable 12 of the flexible circuit electrode array 10 togently lift the flexible circuit cable 12 off the retina and curve itaway from the retina, before it pierces the sclera at a scleratomy. Itis not necessary to heat curve the service loop as described above, theflexible circuit electrode array 10 can simply be bent or creased uponimplantation. This service loop reduces the likelihood of any stressexerted extraocularly from being transmitted to the electrode region andretina. It also provides for accommodation of a range of eye sizes. Withexisting technology, it is necessary to place the implanted controlelectronics outside of the sclera, while a retinal flexible circuitelectrode array 10 must be inside the sclera in order to contact theretina. The sclera must be cut through at the pars plana, forming ascleratomy, and the flexible circuit passed through the scleratomy. Aflexible circuit is thin but wide. The more electrode wires, the widerthe flexible circuit must be. It is difficult to seal a scleratomy overa flexible circuit wide enough to support enough wires for a highresolution array.

FIG. 5 shows a body 1 containing the flexible circuit electrode array10, the flexible circuit cable 12 and the interconnection pad 52 priorto folding and attaching the array to the electronics package 14. At oneend of the flexible circuit cable 12 is an interconnection pad 52 forconnection to the electronics package 14. At the other end of theflexible circuit cable 12 is the flexible circuit electrode array 10.Further, an attachment point 54 is provided near the flexible circuitelectrode array 10. A retina tack (not shown) is placed through theattachment point 54 to hold the flexible circuit electrode array 10 tothe retina. A stress relief 55 is provided surrounding the attachmentpoint 54. The stress relief 55 may be made of a softer polymer than theflexible circuit, or it may include cutouts or thinning of the polymerto reduce the stress transmitted from the retina tack to the flexiblecircuit electrode array 10. The flexible circuit cable 12 is formed in adog leg pattern so than when it is folded at fold 48 it effectivelyforms a straight flexible circuit cable 12 with a narrower portion atthe fold 48 for passing through the scleratomy.

FIG. 6 shows the flexible circuit electrode array 10 after the flexiblecircuit cable 12 is folded at the fold 48 to form a narrowed section.The flexible circuit cable 12 may include a twist or tube shape as well.With a retinal prosthesis as shown in FIG. 1, the interconnection pad 52for connection to the electronics package 14 and the flexible circuitelectrode array 10 are on opposite side of the flexible circuit. Thisrequires patterning, in some manner, both the base polymer layer and thetop polymer layer. By folding the flexible circuit cable 12 of theflexible circuit electrode array 10, the openings for the bond pad 52and the electrodes are on the top polymer layer and only the top polymerlayer needs to be patterned.

Also, since the narrowed portion of the flexible circuit cable 12pierces the sclera, shoulders formed by opposite ends of the narrowedportion help prevent the flexible circuit cable 12 from moving throughthe sclera. It may be further advantageous to add ribs or bumps ofsilicone or similar material to the shoulders to further prevent theflexible circuit cable 12 from moving through the sclera.

Further it is advantageous to provide a suture tab 56 in the flexiblecircuit body near the electronics package 14 to prevent any movement inthe electronics package 14 from being transmitted to the flexiblecircuit electrode array 10. Alternatively, a segment of the flexiblecircuit cable 12 can be reinforced to permit it to be secured directlywith a suture.

An alternative to the bumpers described in FIG. 4, is a skirt ofsilicone or other pliable material as shown in FIGS. 7 to 10. A skirt 60covers the flexible circuit electrode array 10, and extends beyond itsedges. It is further advantageous to include windows 62 adjacent to theattachment point 54 to spread any stress of attachment over a largerarea of the retina. There are several ways of forming and bonding theskirt 60. The skirt 60 may be directly bonded through surface activationor indirectly bonded using an adhesive.

Alternatively, a flexible circuit electrode array 10 may be layeredusing different polymers for each layer. Using too soft of a polymer mayallow too much stretch and break the metal traces. Too hard of a polymermay cause damage to delicate neural tissue. Hence a relatively hardpolymer, such a polyimide may be used for the bottom layer and arelatively softer polymer such a silicone may be used for the top layerincluding an integral skirt to protect delicate neural tissue.

The simplest solution is to bond the skirt 60 to the back side away fromthe retina of the flexible circuit electrode array 10 as shown in FIG.8. While this is the simplest mechanical solution, sharp edges of theflexible circuit electrode array 10 may contact the delicate retinatissue. Bonding the skirt to the front side toward the retina of theflexible circuit electrode array 10, as shown in FIG. 9, will protectthe retina from sharp edges of the flexible circuit electrode array 10.However, a window 62 must be cut in the skirt 60 around the electrodes.Further, it is more difficult to reliably bond the skirt 60 to theflexible circuit electrode array 10 with such a small contact area. Thismethod also creates a space between the electrodes and the retina whichwill reduce efficiency and broaden the electrical field distribution ofeach electrode. Broadening the electric field distribution will limitthe possible resolution of the flexible circuit electrode array 10.

FIG. 10 shows another structure where the skirt 60 is bonded to the backside of the flexible circuit electrode array 10, but curves around anysharp edges of the flexible circuit electrode array 10 to protect theretina. This gives a strong bond and protects the flexible circuitelectrode array 10 edges. Because it is bonded to the back side andmolded around the edges, rather than bonded to the front side, of theflexible circuit electrode array 10, the portion extending beyond thefront side of the flexible circuit electrode array 10 can be muchsmaller. This limits any additional spacing between the electrodes andthe retinal tissue.

FIG. 11, shows a flexible circuit electrode array 10 similar to FIG. 10,with the skirt 60, flush with the front side of the flexible circuitelectrode array 10 rather than extending beyond the front side. Whilethis is more difficult to manufacture, it does not lift the electrodesoff the retinal surface as with the array in FIG. 10. It should be notedthat FIGS. 8, 10, and 11 show skirt 60 material along the back of theflexible circuit electrode array 10 that is not necessary other than forbonding purposes. If there is sufficient bond with the flexible circuitelectrode array 10, it may be advantageous to thin or remove portions ofthe skirt 60 material for weight reduction.

The electrode of the present invention preferably contains platinum.Platinum can be present in any form in the electrode. The electrode haspreferably increased surface area for greater ability to transfer chargeand also having sufficient physical and structural strength to withstandphysical stress encountered in its intended use. The electrode containsplatinum having a fractal configuration so called platinum gray with anincrease in surface area of at least 5 times when compared to shinyplatinum of the same geometry and also having improved resistance tophysical stress when compared to platinum black. Platinum gray isdescribed in U.S. Pat. No. 6,974,533 “Platinum Electrode and Method forManufacturing the Same” to David Zhou, the disclosure of which isincorporated herein by reference. The electrodes of the preferredembodiment are too small to display a color without significantmagnification. The process of electroplating the surface coating ofplatinum gray comprising plating at a moderate rate, i.e., at a ratethat is faster than the rate necessary to produce shiny platinum andthat is less than the rate necessary to produce platinum black.

The flexible circuit electrode array 10 is manufactured in layers. Abase layer of polymer is laid down, commonly by some form of chemicalvapor deposition, spinning, meniscus coating or casting on a supportingrigid substrate like glass. A layer of metal (preferably platinum),preferably sandwich by layers of another metal for example titanium, isapplied to the polymer base layer and patterned to create electrodes andtraces for those electrodes. Patterning is commonly done byphotolithographic methods. The electrodes may be built up byelectroplating or similar method to increase the surface area of theelectrode and to allow for some reduction in the electrode over time.Similar plating may also be applied to the bond pads. See FIGS. 5 to 7.A top polymer layer is applied over the metal layer and patterned toleave openings for the electrodes, or openings are created later bymeans such as laser ablation. It is advantageous to allow an overlap ofthe top polymer layer over the electrodes to promote better adhesionbetween the layers, and to avoid increased electrode reduction alongtheir edges. Alternatively, multiple alternating layers of metal andpolymer may be applied to obtain more metal traces within a given width.

FIG. 12 depicts a supporting rigid substrate 70 which may be marked witha batch and plate identification code by mechanical engraving. Thesupporting substrate 70 is a rigid material like glass.

FIG. 13 shows a polymer layer 71 a which is applied onto the front sideof the supporting substrate 70. The polymer can containpolyfluorocarbons, polyethylene, polypropylene, silicone, polyamide,polyimide, liquid crystal polymer (LCP), poly-para-xylylene (GALXYL®Parylene), polyaryletherketone (Peek®) or other similar polymers. Thepreferred polymer according to the present invention is polyimide.Polyimide is biocompatible and shows excellent properties as insulatorfor the trace metal and electrodes in the electrode assay. Polyimide ispreferably obtained by imidization of polyamic acid. Polyamic acid is aprecursor of the polyimide. By heating the polyamic acid to atemperature of preferably 100° C. to 400° C. the imidization of thepolyamic acid is conducted. The following formula shows an example ofimidization of polyamic acid to polyimide.

R is a bivalent organic group, like 4,4′ oxydianiline orp-phenylendiamine and n is an integer>1. Two polyimides are preferablyused within the present invention. PMDA/ODA (derived from polymelliticdianhydride (PMDA) and 4,4′ oxydianiline (ODA)) and BPDA/PDA (derivedfrom 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) andp-phenylendiamine (PDA)). The polyimide layer 71 a is preferablyobtained according to the present invention with a thickness of theliquid precursor of 1.0 μm to 10 μm, preferably 4.0 μm to 7.0 μm, andmost preferably 5.0 μm to 6.0 μm. The plate is placed in a plasmacleaner. An adhesion promoter is applied on the plate and dried byletting the plate sit for at least 5 min. Spin speed of a spinner andramp up time, spin time, and ramp down time are adjusted and polyamicacid is applied. The plate is soft baked on a hotplate at 90° C. to 110°C. whereby the solvents in the polyimide are evaporated. Then thepolyimide is cured in a high temperature nitrogen purged oven.

FIG. 14 shows that a first thin metal layer 72 a containing titanium,chromium, tantalum or an alloy thereof or a combination of two or morealloys or metal layers thereof, having a thickness of 0.01 μm to 0.1 μm,preferably 0.03 μm to 0.07 μm, and most preferably 0.04 μm to 0.06 μm isapplied on the base polymer layer 71 a preferably by magnetronsputtering. A layer of electrode 73, containing platinum, tantalum,iridium, palladium, rhodium, rhenium, or alloys thereof or a combinationof two or more alloys or metal layers thereof, having a thickness of 0.1μm to 1.0 μm, preferably 0.3 μm to 0.7 μm, and most preferably 0.4 μm to0.6 μm is applied on the thin metal layer 72 a preferably by magnetronsputtering. A top thin layer of metal 72 b, having a thickness of 0.05μm to 0.15 μm, preferably 0.08 μm to 0.13 μm, and most preferably 0.08μm to 0.12 μm, which preferably contains titanium, chromium, tantalum oran alloy thereof or a combination of two or more alloys or metal layersthereof, and particularly the same as the metal layer 72 a, is appliedonto the layer of platinum 73 preferably by magnetron sputteringyielding a thin film stack.

The trace metal of the present invention most preferably contains alower layer of 0.04 μm to 0.06 μm thick titanium film 72 a, a 0.4 μm to0.6 μm thick platinum layer 73 and a 0.08 μm 0.12 μm thin top titaniumfilm 72 b. The present invention provides trace metal with a thin toptitanium film 72 b. The top titanium film 72 b performs an expectedstrong adhesion with the upper polyimide layer 71 b, which is applied asa precursor and is subsequently cured to polyimide. The state of the artteaches to omit a top layer of titanium on top of the platinum layer,because partial oxidation of the titanium surface would weaken theadhesion to polyimide. The present invention has shown a strong andunexpected adhesion a top titanium layer 72 b to polyimide 71 despitethe contrary prior art teaching.

FIG. 15 shows a positive photoresist 74 which is applied on the metaltitanium layer 72 b. The photoresist layer 74 is irradiated by UV lightthrough a mask whereby a pattern is obtained. The irradiated areas ofthe photoresist layer 74 are removed. The removed areas of thephotoresist layer 74 masked selectively the areas where vias and tracesare obtained. In this process the plate front side is first dehydrated.A mask aligner is turned on prior to use. The spin speed of a spinner isadjusted to 1700 rμm to 1900 rpm. The ramp up time, spin time, and rampdown time are adjusted to 3 s to 7 s, 15 s to 25 s, and 3 s to 7 s.Positive photoresist is poured onto the center of the plate to form apuddle that is 5.8 cm to 6.6 cm in diameter. The plate is soft baked ona hot plate at 100° C. to 110° C. for 25 s to 35 s. The requiredexposure dose is 80 mJ/cm²±5 mJ/cm² at 436 nm. The plate is alignedunder a mask pattern in the mask aligner. The plate is exposed in themask aligner. The plate is developed in developer for 40 s to 50 s(seconds) with manual agitation of a carrier boat. The photoresist isdissolved during development. The exposed photoresist is dissolvedduring the development. Then the plate is first rinsed in a lowercascade rinse bath for 50 s to 70 s then in a middle cascade rinse bathfor 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to70 s. The plate is dried with filtered nitrogen.

FIG. 16 shows how the thin top layer 72 b is removed in the exposedareas. Typically the preferred titanium is removed at 20° C. to 30° C.for 50 s to 70 s with dilute hydrofluoric acid in a container. The plateis loaded into a carrier boat before etching the titanium. The plate isfirst rinsed in a lower cascade rinse bath for 50 s to 70 s then in amiddle cascade rinse bath for 50 s to 70 s and finally in a bubblercascade rinse bath for 50 s to 70 s. The plate is dried with filterednitrogen, front side only.

FIG. 17 shows platinum being removed from the layer 73 by wet etch inthe exposed areas. The plate is loaded into a carrier boat for removingplatinum. Typically the preferred platinum is removed with Aqua Regia,which is a mixture of hydrochloric acid and nitric acid and water, at60° C. to 70° C. for 7 min to 8 min to 10 min with continuous automaticagitation by 5 cm stir bar rotating at 400 rpm to 600 rpm. The plate isfirst rinsed in a lower cascade rinse bath for 50 s to 70 s then in amiddle cascade rinse bath for 50 s to 70 s and finally in a bubblercascade rinse bath for 50 s to 70 s. The plate is dried with filterednitrogen, front side only.

FIG. 18 shows the residual photoresist layer 74 being removed with aliquid immersion solvent. The plate is soaked in acetone for 1 min to 3min with manual agitation of the carrier boat at least every 15 s to 25s. Then the plate is soaked in isopropanol for 1 min to 3 min withmanual agitation of the carrier boat at least every 15 s to 25 s.

FIG. 19 shows that metal is removed from the thin titanium layer 72 a bywet etch in the masked areas. Typically the preferred titanium isremoved at 20° C. to 30° C. for 30 s to 40 s with dilute hydrofluoricacid. The plate is first rinsed in a lower cascade rinse bath for 50 sto 70 s then in a middle cascade rinse bath for 50 s to 70 s and finallyin a bubbler cascade rinse bath for 50 s to 70 s. The plate is driedwith filtered nitrogen, front side only. The plate is then dehydrated inpreheated oven at 110° C. to 130° C. for 8 min to 10 min.

FIG. 20 shows that the areas where titanium 72 a and 72 b and platinum73 were removed are not covered by trace metal conductors but are openand base polyimide 71 a is the surface layer. Base polyimide surfacelayer 71 a is then activated by RIE in all areas not covered by tracemetal conductors. The plate is loaded in this process into a plasmacleaner. The plate is cleaned in 400 mTorr to 600 mTorr O₂ for 4 min to6 min at 180 W to 220 W (W stands for Watt). Then the plate is treatedin KOH-deimidization bath at 20° C. to 30° C. for 4 min to 6 min withmanual agitation of the carrier boat at least every 50 s to 70 s. Theplate is first rinsed in a lower cascade rinse bath for 50 s to 70 sthen in a middle cascade rinse bath for 50 s to 70 s and finally in abubbler cascade rinse bath for 50 s to 70 s. The plate is dried withfiltered nitrogen, front side only. The plate is treated in anHCl-deimidization bath at 20° C. to 30° C. for 4 min to 6 min withmanual agitation of the carrier boat at least every 50 s to 70 s. Theplate is rinsed in the lower cascade rinse bath for 50 s to 70 s. Theplate is rinsed in the middle cascade rinse bath for 50 s to 70 s. Theplate is rinsed in the bubbler cascade rinse bath for 50 s to 70 s. Theplate is dried with filtered nitrogen, front side only. The plate issoaked in isopropanol at 20° C. to 30° C. for 1 min to 2 min with manualagitation of the carrier boat at least every 15 s to 25 s. The plate isdried with filtered nitrogen, front side only. The plate is dehydratedunder vacuum pressure of less than 100 mTorr for at least 2 hr.

The activation process according to the present invention provides asurface of the base layer 71 a which is deimidized and therefore thesurface is similar to the uncured state. The imidization processobtained during curing is now reversed.

The activated surface layer of the base polyimide layer is chemicallyvery similar to the precursor of the top polyimide layer 71 b which isapplied to yield a top layer. When the precursor of the top layer 71 bis applied on the activated and deimidized base layer 71 a then there isno boundary between the two layers in the contact area.

FIG. 21 shows' how a top layer of polymer 71 b is applied onto the topof base polymer layer 71 a. The polymer contains polyfluorocarbons,polyethylene, polypropylene, silicone, polyamide, polyimide, liquidcrystal polymer (LCP), poly-para-xylylene (GALXYL® Parylene),polyaryletherketone (Peek®) or other similar polymers. Polyimide showsexcellent properties as an insulator for the trace metal and electrodesin the electrode array. Polyimide is preferably obtained by imidizationof polyamic acid. Polyamic acid is a precursor of the polyimide. Theimidization of the polyamic acid to polyimide is conducted by heatingthe polyamic acid at a temperature of preferably 100° C. to 400° C. Thepolyimide layer 71 b is preferably obtained according to the presentinvention with a thickness of 1.0 μm to 10 μm, preferably 4.0 μm to 7.0μm, and most preferably 5.0 μm to 6.0 μm. The plate is placed in thisprocess in a plasma cleaner. An adhesion promoter is applied on theplate and dried off by letting the plate sit for at least 5 min. Spinspeed of a spinner and ramp up time, spin time, and ramp down time areadjusted and polyamic acid is applied. The plate is soft baked on ahotplate at 100° C. to 110° C. whereby the solvents in the polyimide areevaporated. The polyimide is cured in a high temperature nitrogen purgedoven.

FIG. 22 shows that base polyimide layer 71 a and top polyimide layer 71b become one polyimide layer 71 performing a high adhesion to the tracemetal. An insulation mask of aluminum thin film 75 is applied preferablyby magnetron sputtering on top of the polyimide layer 71 having athickness of 0.5 μm to 1.5 μm, preferably 0.7 μm to 1.3 μm, and mostpreferably 0.9 μm to 1.1 μm.

FIG. 23 shows that a positive photoresist 76 is applied on the aluminumlayer 75. The photoresist layer 76 is irradiated by UV light through amask whereby a pattern is obtained. The irradiated areas of thephotoresist layer 76 are removed by a solvent. The removed areas of thephotoresist layer 76 expose selectively the areas where vias are to beobtained in the further process. The plate front side is dehydrated at110° C. to 130° C. for 15 min to 25 min. A mask aligner is turned onprior to use. The spin speed of a spinner is adjusted to 2800 rμm to3200 rpm. The ramp up time, spin time, and ramp down time are adjustedto 15 s to 25 s, 1 s to 2 s, and 2 s to 4 s respectively. Positivephotoresist 76 is poured onto the center of the plate to form a puddlethat is 3.6 cm to 4.0 cm in diameter. The plate is soft baked on a hotplate at 110° C. to 120° C. for 4 min to 5 min. The required exposuredose is 470 mJ/cm² to 490 mJ/cm² at 436 nm. The Insulation Viasphotomask is used in the aligner. The plate is aligned under maskpattern in the mask aligner. The plate is exposed in the mask aligner.The plate is developed in a developer for 40 s to 50 s with manualagitation of the carrier boat. The plate is developed in heateddeveloper mixture at 20° C. to 30° C. for 4 min to 6 min with manualagitation every 30 s. The plate is first rinsed in a lower cascade rinsebath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70s and finally in a bubbler cascade rinse bath for 50 s to 70 s. Theplate is dried with filtered nitrogen, front side only. The plate ishard baked in oven for 25 min 35 min at 80° C. to 100° C.

FIG. 24 shows that the aluminum layer 75 is removed in exposed areas bywet etch. The plate is plasma cleaned in this process in 500 mTorr to600 mTorr O₂ for 4 min to 6 min at 180 W to 220 W. The plate is treatedin heated Al Etch, containing 71% to 73% phosphoric acid, 9% to 11%acetic acid, and 1% to 3% nitric acid, at 30° C. to 40° C. for 9 min to12 min with manual agitation every 50 s to 70 s. The plate is firstrinsed in a lower cascade rinse bath for 1 min to 3 min then in a middlecascade rinse bath for 1 min to 3 min and finally in a bubbler cascaderinse bath for 1 min to 3 min. The plate is dried with filterednitrogen, front side only. The plate is dehydrated for 25 min to 35 minat 90° C. to 110° C.

FIG. 25 shows that polyimide surface layer 71 is removed by RIE(Reactive Ion Etching) in all areas not covered by the aluminum layer75.

FIG. 26 shows that the residual photoresist layer 76 was removed fromthe aluminum layer 75 with a liquid immersion solvent. The plate issoaked in acetone for 1 min to 3 min with manual agitation of thecarrier boat at least every 15 s to 25 s. Then the plate is soaked inisopropanol for 1 min to 3 min with manual agitation of the carrier boatat least every 15 s to 25 s. The plate is first rinsed in a lowercascade rinse bath for 50 s to 70 s then in a middle cascade rinse bathfor 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to70 s. The plate is dried with filtered nitrogen, front side only.

FIG. 27 shows that remaining insulation mask aluminum layer 75 isremoved from the polyimide 71 layer by wet etch. The plate is plasmacleaned in 550 mTorr O₂ for 4 min to 6 min at 180 W to 220 W. The plateis treated in heated Al Etch at 30° C. to 40° C. for 9 min to 11 minwith manual agitation every 50 s to 70 s. The plate is first rinsed in alower cascade rinse bath for 1 min to 3 min then in a middle cascaderinse bath for 1 min to 3 min and finally in a bubbler cascade rinsebath for 1 min to 3 min. The plate is dried with filtered nitrogen,front side only. The plate is dehydrated for 25 min to 30 min at 100° C.to 110° C.

FIG. 28 shows that the titanium 72 b top surface layer is removed in thevias by wet etch to leave platinum 73 as the top surface layer in thevias. Typically the preferred titanium is removed at 20° C. to 30° C.for 50 s to 70 s with dilute hydrofluoric acid. The plate is firstrinsed in a lower cascade rinse bath for 50 s to 70 s then in a middlecascade rinse bath for 50 s to 70 s and finally in a bubbler cascaderinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen,front side only.

FIG. 29 shows that the arrays are singulated by cutting through thepolyimide 71 layer by laser 77 whereby individual parts are created. Thesingle circuit electrode array is then removed from the supporting glassplate substrate 70 by a solvent.

FIG. 30 shows that the supporting glass substrate 70 is removed. Thevias are electroplated with preferably platinum, most preferablyplatinum gray 78.

The vias are preferably filled with platinum gray 78 are electroplatedon the platinum trace 73. Platinum gray is described in U.S. Pat. No.6,976,998 “Platinum Electrode and Method for Manufacturing the Same” toDavid Zhou, the disclosure of which is incorporated herein by reference.The method to produce platinum gray according to the present inventioncomprises connecting a platinum electrode, the anode, and a conductivesubstrate to be plated, the cathode, to a power source with a means ofcontrolling and monitoring either the current or voltage of the powersource. The anode, cathode, a reference electrode for use as a referencein controlling the power source and an electroplating solution areplaced in a electroplating cell having a means for mixing or agitatingthe electroplating solution. Power is supplied to the electrodes withconstant voltage, constant current, pulsed voltage, scanned voltage orpulsed current to drive the electroplating process. The power source ismodified such that the rate of deposition will cause the platinum todeposit as platinum gray, the rate being greater than the depositionrate necessary to form shiny platinum and less than the deposition ratenecessary to form platinum black.

The electrode region of the array 10 is immersed in this process intosulfuric acid. The electrode region of the array 10 is rinsed under aflow of distilled water for at least 30 s. The plating solution is keptat 21° C. to 23° C. The plating solution is being stirred at 180 rμm to220 rpm. The electrical contact fixture is loaded into the rotaryholder. The electrode end of the array 10 is immersed into plating bathand the apparatus is aligned. The system is allowed to stabilize for 4min to 6 min. After starting a potentiostat program a pneumatic controltimer is started immediately. The array is rotated by 90° every 4 min to6 min for the duration of plating. After the plating cycle is completed,the electrode region of the array 10 is rinsed under a flow of distilledwater for at least 30 s. The array is then removed from theelectroplating electrical contact fixture. The entire array is rinsed ina vial with fresh distilled water for up to about 24 hours. The array 10is then dried in nitrogen.

It becomes apparent especially from FIG. 30 that the electrode 78,preferably platinum gray has a direct contact to the conducting layer73, preferably platinum. The electrode 78 is slightly slimmer than thetrace 72 a/73/72 b, preferably titanium/platinum/titanium. Polymer 71,preferably polyimide, covers the part of the trace, which does not havea contact with the electrode 73. This part of the trace is covered bytitanium 72 b. There is a surprisingly strong adhesion between titanium72 b and polyimide 71. The trace 72 a/73/72 b is referred to in FIG. 30and subsequently in FIGS. 31-32 simplified as number 79.

FIG. 31 shows an enlarged top view of the flexible circuit electrodearray 10 which is a part of the body 1 as shown for example in FIG. 5.The preferred positions of the electrodes 78 and the preferred wiring bythe trace metal 79 both embedded in the polymer 71 are shown in the FIG.31.

FIG. 32 shows a three dimensional view of a part of the flexible circuitelectrode array 10. It shows one electrode 78 which has a contact withthe trace metal 79. It also shows that the trace metal 79 overlaps theelectrode 78 and the electrode 78 overlaps the via in the polymer 71.The FIG. 32 further shows the adhesion of the polymer 71 with the tracemetal 79 and the electrode 78 which results in a very high effectiveinsulation of the trace metal 79 and the electrode 78. The figure showsalso that the trace metal 79 is preferably composed of platinumconducting trace 73 covered on the lower and upper side preferably witha thin titanium layer 72 a and 72 b. The figure finally shows that thefirst applied base polymer 71 a and the subsequently applied top polymerlayer 71 b form a single polymer layer 71.

The present invention will be further illustrated by the followingexamples, but it is to be understood that the invention is not meant tobe limited to the details described herein.

EXAMPLE 1

A 10.2 cm×10.2 cm×0.15 cm supporting glass plate substrate 70 was markedwith a batch and plate identification code by mechanical engraving. Thena 5.5 μm thick layer of polyimide BPDA/PDA (derived from3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) andp-phenylendiamine (PDA)) layer 71 a was applied onto the front side ofthe glass plate 70 as a liquid precursor by spin coating and cured toPolyimide, PI2611.

Then a 0.05 μm layer of titanium 72 a was applied on the polyimide layer71 a preferably by magnetron sputtering, a 0.5 μm layer of platinum 73was applied on the titanium layer 72 a preferably by magnetronsputtering, and a 0.10 μm layer of titanium 72 b was applied onto thelayer of platinum 73 preferably by magnetron sputtering yielding atitanium/platinum/titanium thin film stack.

Subsequently a 2 μm layer 74 of positive photoresist AZ 1512(Microchemicals GmbH, Germany) was applied on the titanium layer 72 b.The photoresist layer 74 was irradiated by UV light through a maskwhereby a pattern was created. The irradiated areas of the photoresistlayer 74 where removed by a developer solution AZ 300-MIF(Microchemicals GmbH, Germany). The remaining areas of the photoresistlayer 74 masked selectively the areas where metal traces were obtainedin the further process.

Titanium from the layer 72 b was removed by wet etch, 50:1 Dilute HF(Ashland Chemical), in the exposed areas. Platinum from the layer 73 wasremoved by wet etch, DI water:HCl:HNO₃ in volume ratio 1:3:1, in theexposed areas. Then the residual photoresist layer 74 was removed with aliquid immersion 20 s in acetone and 20 s in isopropanol. Titanium fromthe layer 72 a was removed by wet etch, 50:1 Dilute HF (AshlandChemical), in the exposed areas. The areas where titanium 72 a and 72 band platinum 73 were removed expose polyimide 71 as the surface layer.

The base polyimide surface layer 71 a was activated and partiallyremoved by RIE in all areas not covered by trace metal conductors. Thesurface was treated in 100 mTorr, 85% O₂, 15% CF₄ for 2 min at 200 W and20° C. as shown in the following table 1.

TABLE 1 Reactive Iron Etch (RIE) Pressure Power Time Temperature StepGases [mTorr] [W] [m:s] [° C.] Pump Down — 1 0 00:01 20 Etch 85% O₂, 15%CF₄ 100 200 02:00 20 Pump Down — 1 0 00:01 20

The surface was subsequently treated in KOH-deimidization bath at 25° C.for 5 min with manual agitation of the carrier boat at least every 60 s.The surface was first rinsed in a lower cascade rinse bath for 60 s, ina middle cascade rinse bath for 60 s, and finally in a bubbler cascaderinse bath for 60 s. The surface was dried with filtered nitrogen. Thesurface was then treated in an HCl-deimidization bath at 25° C. for 5min with manual agitation of the carrier boat at least every 60 s. Thesurface was first rinsed in a lower cascade rinse bath for 60 s, in amiddle cascade rinse bath for 60 s, and finally in a bubbler cascaderinse bath for 60 s. The deimidization process is shown in the followingtable 2.

TABLE 2 Deimidization Step Concentration Pressure [Torr] Time [m:s]Temperature [° C.] KOH 1.0 N KOH 1 5:00 25 HCl 1.0 N HCl 1 5:00 25

Then a 5.5 μm thick top layer 71 b of a precursor solution was appliedby spin coating and cured to Polyimide, PI2611 on the top of basepolyimide 71 a. Polyimide 71 a and 71 b combined to polyimide 71 aftercuring.

Then an adhesion promoter, VM652, and aluminum foil, P/N X11652-1.518(All Foils Inc) were applied by magnetron sputtering on top of thepolyimide layer 71 yielding 1.0 μm mask aluminum thin film 75.

Then a 12 μm layer of positive photoresist 76, AZ P4620 (MicrochemicalsGmbH, Germany) was applied on the aluminum layer 75. The photoresistlayer 76 was irradiated by UV light through a mask whereby a pattern wascreated. The irradiated areas of the photoresist layer 76 were removedby a developer AZ 1:1 (Microchemicals GmbH, Germany) [Developer: DIWater]. The removed areas of the photoresist layer 76 exposedselectively the areas where vias were obtained in the further process.

Aluminum 75 was removed in exposed areas by wet etch, sulfuric acid, 1.0N (0.5 M). The polyimide surface layer 71 was removed by RIE in allareas not masked by aluminum. Then the residual photoresist layer 76 wasremoved from the aluminum layer 75 by immersion 20 s in acetone and 20 sin isopropanol. Vias were created with titanium 72 b as the top surfacelayer in the vias. Then the aluminum mask layer 75 was removed from thepolyimide 71 layer by wet etch. The titanium 72 b top surface layer wasremoved in the vias by wet etch, 50:1 Dilute HF (Ashland Chemical), toleave platinum 73 as the top surface layer in the vias.

The arrays were singulated by cutting through the entire polyimide 71layer by laser to create individual parts prior to removal from thesupporting glass plate substrate 70. The array 10 was removed from thesupporting glass substrate 70.

The electrodes were electroplated with platinum gray 78. The platingsolution contained 18 mM (NH₄)₂PtCI₆ dissolved in 0.46 M Na₂HPO₄. Theelectrode vias were filled with platinum gray 78, which was in contactwith the platinum trace 73.

EXAMPLE 2

Example 2 was carried out according to example 1 with the differencethat the base polyimide surface layer 71 a was activated and partiallyremoved by RIE in all areas not covered by trace metal conductors. Thesurface was treated in 100 mTorr, 85% O₂, 15% CF₄ for 2 min at 200 W and20° C. as shown in the preceding table 1 and the deimidization wasomitted. The adhesion strength between the base polyimide layer 71 a andthe top polyimide layer 71 b is shown in table 3.

TABLE 3 Adhesion Strength Polyimide - Polyimide Adhesion Polyimide -Polyimide Strength [N] Adhesion Strength [N] Ex RIE Deimidization DryWet 1 85% O₂, 3.0 2.4 15% CF₄

Table 3 shows the measurement of two adhered dry polyimide films and twoadhered polyimide films kept in saline solution for 7 days at 87° C.

EXAMPLE 3

Example 3 was carried out according to example 1 with the differencethat the base polyimide surface layer 71 a was activated bydeimidization. The surface was subsequently treated in KOH-deimidizationbath at 25° C. for 5 min with manual agitation of the carrier boat atleast every 60 s. The surface was first rinsed in a lower cascade rinsebath for 60 s, in a middle cascade rinse bath for 60 s, and finally in abubbler cascade rinse bath for 60 s. The surface was dried with filterednitrogen. The surface was then treated in an HCl-deimidization bath at25° C. for 5 min with manual agitation of the carrier boat at leastevery 60 s. The surface was first rinsed in a lower cascade rinse bathfor 60 s, in a middle cascade rinse bath for 60 s, and finally in abubbler cascade rinse bath for 60 s. The deimidization process is shownin the following table 2 and the RIE was omitted. The adhesion strengthbetween the base polyimide layer 71 a and the top polyimide layer 71 bis shown in table 4. Table 4 shows the measurement of two adhered drypolyimide films (DRY) and two adhered polyimide films kept in salinesolution for 7 days at 87° C. (WET).

TABLE 4 Adhesion Strength Polyimide - Polyimide - Polyimide PolyimideAdhesion Adhesion Strength [N] Ex RIE Deimidization Strength [N] DRY WET1 KOH, HCl 2.1 2.0

While the invention has been described by means of specific embodimentsand applications thereof, it is understood that numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the spirit and scope of the invention. It is therefore tobe understood that within the scope of the claims, the invention may bepracticed otherwise than as specifically described herein.

EXAMPLE 4

Example 4 was carried out according to example 1 with the differencethat the polyimide layers were cured at lower temperature as shown intable 5 bellow.

The polyimide layer was treated in an initial bake, a soft-bake, for 10minutes at 100° C. on a hot plate. The polyimide layer was then purgedin a nitrogen oven ramp at 4° C./min to 325° C., and held for 30 min at325° C. and then ramped down.

The polyimide layers were treated according to example 1 in a soft-bakefor 10 minutes at 100° C. on a hot plate. The polyimide layer was thenpurged in a nitrogen oven ramp at 4° C./min to 375° C., and held for 30min at 375° C. and then ramped down.

TABLE 5 Curing of Polyimide Example 1 Example 4 Time Temp. Time Temp.Step Phase [h:m] [° C.] [h:m] [° C.] 1 Purge 0:30 30 0:30 30 2 InitialBake 0:10 100 0:10 100 3 Ramp up 2:00 375 2:00 325 4 Dwell 0:20 375 0:20325 5 Ramp down 3:00 30 3:00 30 Damper ½ open N₂ Line 40-45 psi PressureN₂ Throat  20 SCFH Flow N₂ Purge 100 SCFH Flow N₂ un Flow  30 SCFH

The curing parameters according to example 1 lead to a polyimide with alow permeability. The lower temperature applied in example 4 leads to ahigher but sufficient permeability. Further, polyimide layer preparedaccording to example 4 yields higher flexibility which advantageouslyminimizes the possibility of damaging to the retina.

1. A method for manufacturing a flexible circuit electrode array,comprising the steps of: depositing a metal trace layer on an insulatorpolymer base layer having a surface; applying a photoresist layer onsaid metal trace layer and patterning said metal trace layer and formingmetal traces on said insulator polymer base layer; activating saidinsulator polymer base layer and depositing a top insulator polymerlayer and forming an insulating polymer layer with said insulatorpolymer base layer; heating said insulator polymer base layer at between80-150° C. and then between 230-350° C.; wherein activating saidinsulator polymer base layer by etching said surface of said insulatorpolymer base layer by reactive ion etching and deimidizing said surfaceof said insulator polymer base layer.
 2. A method for manufacturing aflexible circuit electrode array, comprising the steps of: depositing ametal trace layer on an insulator polymer base layer having a surface;applying a photoresist layer on said metal trace layer and patterningsaid metal trace layer and forming metal traces on said insulatorpolymer base layer; activating said insulator polymer base layer anddepositing a top insulator polymer layer and forming an insulatingpolymer layer with said insulator polymer base layer; heating saidinsulator polymer layer at between 80-150° C. and then between 230-350°C.; wherein deimidizing said surface of said insulator polymer baselayer by sequential treatments with KOH and HCl.